1. Field of the Invention
This invention relates to a control system to control a dryer utilized for the drying of plastic pellets that are ultimately melted and used to form plastic articles of manufacture. The control system is designed to simplify operation of the pellet dryer and to utilize readily available components to provide a reliable control system.
2. Description of the Prior Art
Dryers and dryer hoppers have been utilized in the plastic industries for many years. These dryers provide heated, dehumidified air to a dryer hopper. Plastic pellets within the dryer hopper are exposed to the heated, dehumidified air to remove moisture from the pellets. The basic purpose of drying the plastic pellets is to remove moisture from the surface and interior of the pellets before the pellets reach the melt phase as they pass through the screw area of an injection or extrusion machine. If the moisture is not removed, process problems may occur or the quality of the finished parts may be inferior to parts which were made with properly dried material. Some properties of the parts which may be affected by inadequate drying are: tensile strength; impact strength; surface blemishes; and degradation of barrier properties.
Moisture may be present either on the surface of the plastic pellet or in the polymer chain itself. Hygroscopic resin absorbs moisture into the polymer chain itself. Other resins hold only surface moisture. Proper heating and dehumidifying of the pellet causes the moisture to be driven out of hygroscopic resins onto the surface. Surface moisture is removed by the heated, dehumidified air.
It is well known in the art to utilize a generally cylindrical dryer hopper having a frusto-conical base to house plastic pellets for drying. The heated and dehumidified air is admitted to the lower portion of the dryer hopper and passes upwardly through the dryer hopper containing the pellets and is removed at the top of the hopper. The air, after having passed through the pellets, contains moisture and is substantially cooler than when it entered the hopper. This cooler, moisture-laden air is then recycled through a process cooler to further cool the air and then passes through a desiccant section where moisture is removed from the air. The air then travels through a process heater where it is heated again to a temperature sufficient to dry the pellets and that air is passed through the dryer hopper. The desiccant section typically contains three or more desiccant tanks containing molecular sieves usually formed of Alumino-silicates which remove the moisture from the air. The desiccant tanks are rotated so that while one is in use, the others are being regenerated to accept moisture and remove it from the air.
Various control systems have been utilized to control the flow of air through the dryer hopper, to control the operation of the desiccant regeneration feature and to control the overall drying of the plastic pellets within the dryer hopper. United States patent 4,413,426 is an example of an effort to control the drying process within a dryer hopper by controlling the temperature of the exhaust air from the hopper.